1. Field of the Invention
This invention relates generally to electronic devices, and particularly to packaging of such devices. More specifically, the invention provides a solder shape array package for high pin count integrated circuit devices.
2. Background Art
Advances in semiconductor processing technology have provided devices of ever increasing complexity. Generally speaking, more complex integrated circuit devices have a greater number of input/output (I/O) signals. Consequently, the increasing complexity of integrated circuits poses a challenge in terms of packaging. Aside from the problem of providing a sufficient number of circuit leads to meet the I/O requirements of a particular device, there is the attendant problem of properly bonding a large number of leads to a printed circuit board. Additionally, it is desirable to bond the device to the printed circuit board in a manner that will facilitate subsequent removal for replacement of the device.
Pin grid array packages have become a popular form of integrated circuit packaging since they provide a large number of pins in a plug-in type package. An array of pins is located on a ceramic or plastic substrate and are received by mating sockets on a printed circuit board. The integrated circuit or other semiconductor device is mounted on the top side of the substrate and electrical bonds are made to the individual pins, typically with wire bonding. Maintaining mechanical alignment of the pins and achieving reliable electrical contact between the pins and mating sockets are problems associated with pin grid array packages.
Another type of package for high pin count devices is the pad grid or bump grid package. In this type of package, conductive pads or bumps on the underside of the chip carrier substrate are provided in lieu of pins. The chip carrier substrate in this type of package must meet a stringent flatness requirement in order to ensure that all of the pads make reliable electrical contact with the receiving printed circuit board.
The ball grid array (BGA) package is similar in concept to the pad grid array, but achieves greater mounting compliance by attaching a small solder ball at each of the pad sites. This has proven to be a reliable packaging technique, but poses certain fabrication difficulties. Current surface mount BGA packages are constructed by placing individual solder balls onto an array of substrate pads on which flux and/or solder paste has been deposited. The substrate is then exposed to a temperature sufficient to melt the solder, thereby attaching the individual solder balls to their respective pads. Properly locating the individual solder balls at the pad sites is a difficult process to automate and results in a relatively high-cost package.
The present invention presents an alternative to existing BGA technology by providing a method for constructing a solder shape array surface mount package with the mounting characteristics of a BGA package, but at significantly lower cost.